1. Field of the Invention
The present invention relates to a fail-safe signal transmitting apparatus and a constant voltage power supply. More specifically, the present invention relates to a fail-safe signal transmitting apparatus which ensures that no error is generated in the output transmission signal which would allow a dangerous situation to arise, even when a multiple failure has occurred in circuits that include the power supply and that constitute a signal transmitting apparatus.
2. Discussion of the Background
In areas such as railway technology, press control, aircraft control technology and nuclear power technology which require a high degree of safety, a completely fail-safe signal transmitting apparatus, which operates in support of its safety function without error in the case of a circuit failure is absolutely necessary. Fail-safe signal processing technology is disclosed in publications such as, U.S. Pat. No. 4,661,880, U.S. Pat. No. 5,027,114, U.S. Pat. No. 5,345,138, Japanese Examined Patent Publication No. 23006/1989 and Japanese Examined Patent Publication No. 2948/1993. By adopting the technology disclosed in these publications of prior art, signal transmission can be fail-safe under limited conditions. However, these publications of prior art do not disclose a means for securing fail-safe transmission in case of a circuit failure in the signal transmitting apparatus accompanied by a failure in a constant voltage power supply that delivers power to the signal transmitting apparatus.
Normally, a commercially available constant voltage power supply is provided with an excess current detector and a protection circuit that shuts down the output current if an excess current should be supplied to the load. Such a constant voltage power supply is provided with a constant voltage circuit which may be a so-called series regulator. However, in a constant voltage power supply provided with an excess current protection circuit, there is no provision for a failure mode that will disable the function for cutting off excess current when there is a failure in the excess current detection circuit. A fail-safe source monitoring apparatus, which cuts off the output current from the constant voltage power supply or the output from the processing apparatus that uses the output of the source voltage when a failure occurs in the excess current protecting apparatus, does not exist in the prior art. In addition, a fail-safe source monitoring apparatus that cuts off the output if there is a failure in the constant voltage power supply does not exist in the prior art.
This means that even when the signal transmitting apparatus itself has a fail-safe circuit structure, the fail-safe aspect of the entire signal transmitting apparatus including the power supply is lost in case of a circuit failure in the power supply.